Thread: How to analyze a string for numbers or other characters

Originally Posted by chickenlittle

I am trying to write a program that can do postfix expressions in c++. First I have to read in the user input as a string. But how would I break up the string so I can tell whether each data is a number or a math operator? Please help me with this.

Code:

int main()
{
string a;
getline(cin, a)
//How to break up the string into individual chunks for analyzing?
return 0;
}

Think of the problem you are trying to solve: You have a string of n elements, for each element you want to check whether it matches any one of these [0,1,2,3,4,5,6,7,8,9] or if not, whether it matches anyone of these [+, -, *, /].

You would probably want to put the code for this into a function, and you should probably take into account what the function should return, and especially what it should return if it runs into a string element who doesn't match any of the above (Like a whitespace character or a letter or whatever). I would suggest calling the function isNum(), then you could return true if it is a match to the first list of characters, if not, return false if it is a match to the second list of characters, and possibly throw an exception if an invalid element was found.

Give it a try and show us what you come up with.

Originally Posted by chickenlittle

But how would you check to see if the string contains numbers or math operators? That is where I am stuck in. Would we use stringstream?

I tried to write out the problem in terms that could be almost directly translated to C++:

You have a string of n elements, for each element you want to check if it equals any one of these [0,1,2,3,4,5,6,7,8,9] or else, if it equals anyone of these [+, -, *, /]

You see now? This is probably as close to writing out the program logic in pseudocode, as possible, and that wouldn't teach you anything at all. Now, write some code. If it doesn't work, show it to us.

Originally Posted by Shingetsu Kurai

It would be more worthwhile to check if it's an operator, and if not, assume it's a number.

Ok, good so far....

Originally Posted by Shingetsu Kurai

I'll assume you have a vector<string> named origin that contians all your strings.

Why? Let's keep things on task for the OP shall we. He specifically stated he had a string, not a vector of strings.

@OP: The std::string can be manipulated char by char by iterating through it:

Code:

for(int i =0; i<str.size();i++){
     std::cout<<str[i]<<std::endl;
}

Just simply apply your shunting yard algorithm to each char of the string.